In a wireless communications system, a base station system includes a baseband unit (BBU) and a remote radio unit (RRU). A power amplifier (PA) is the most important active device in a remote radio unit of a radio transmitter, and a function of the power amplifier is to amplify an input signal. Efficiency of the PA is an important factor that affects costs of a radio transmitter. However, the PA is not completely ideal. When an amplitude of an input signal exceeds a linear amplification range of the PA, the PA introduces a non-linear distortion, that is, a non-linear amplification relationship between an output signal and the input signal. To ensure linearity of the PA, power back-off needs to be performed on the input signal, and a value of the power back-off depends on a peak-to-average ratio of a signal. A peak-to-average ratio is a ratio of a signal peak power to an average power. For a same PA, because a gain of the PA is constant, a higher output power indicates higher efficiency of the PA. Therefore, to ensure linearity of the PA, for a higher peak-to-average ratio of a signal, a higher level of power back-off needs to be performed, which causes a lower output power. When a peak-to-average ratio of a signal is decreased, efficiency of the PA may be indirectly improved. Therefore, a peak-to-average ratio of a signal is an important parameter for the radio transmitter.
For a communications system that uses a multi-carrier technology (such as orthogonal frequency division multiplex (OFDM)), a peak-to-average ratio of a baseband signal is high. A conventional method for decreasing a peak-to-average ratio of a signal is to use a crest factor reduction (CFR) technology. As shown in FIG. 1, a peak-to-average ratio of an original signal is A (that is, a value obtained by dividing a peak power by a signal average power), some signals whose amplitudes are relatively high are located in a non-linear amplification zone. After a clipping technology is used, a peak-to-average ratio is decreased (as shown in B in FIG. 1). In this case, all signals are located in a linear amplification zone. Further, an average power of an input signal may be increased, so that a sum of the peak power and the gain is still equal to a saturation point, thereby improving efficiency of the PA. However, the CFR technology increases an error vector magnitude (EVM) of a signal, and an increased EVM indicates that a signal cannot be easily demodulated at a receive end. That is, the CFR technology decreases a peak-to-average ratio of a signal at the cost of the EVM. If the EVM is excessively high, it cannot be ensured that data symbols obtained by using various modulation schemes can be properly demodulated at a receive end.
Currently, a widely used PC-CFR (Peak Cancellation CFR) algorithm is an effective algorithm for decreasing a peak-to-average ratio of a signal. A block diagram of the algorithm is shown in FIG. 2. A high peak-to-average ratio signal is input; peak detection is performed on the signal; a peak amplitude and phase information are output to a peak forming module; and a peak location is output to an allocation module. The peak forming module outputs a peak forming factor that may be represented as α=(|x|−γ)×exp(jθ). |x| is an amplitude of a peak signal point, γ is a clipping threshold, and θ is a phase of the peak signal point. The allocation module allocates a cancellation pulse generator (CPG) to a detected peak. The CPG generates a cancellation pulse, the cancellation pulse is pre-stored, and design of the cancellation pulse depends on such information as a bandwidth of a signal. A quantity of CPGs may vary. A sum of products obtained by separately multiplying cancellation pulses corresponding to multiple peaks by peak forming factors corresponding to all peaks is referred to as a clipping noise. After the clipping noise is subtracted from the input high peak-to-average ratio signal that is properly delayed, a signal with a decreased peak-to-average ratio is output. Generally, after the foregoing iteration is performed for two or three times, a preset target peak-to-average ratio of an output signal may be obtained.
In an OFDM system, each resource block (that is, a time-frequency resource that includes OFDM symbols and subcarriers) may transmit data obtained by using different modulation schemes. However, the PC-CFR algorithm uses a constant cancellation pulse that has a consistent amplitude-frequency response on a band. Therefore, after clipping processing is performed on data obtained by using various modulation schemes, obtained EVMs are almost the same. For different modulation schemes, a receive end has different EVM requirements while proper demodulation of data is ensured, and a same EVM is unconducive to further improving efficiency of the PA.